This invention relates to an optical glass of a SiO.sub.2 --TiO.sub.2 --K.sub.2 O--RO (where R represents Mg, Ca, Sr, Ba and Zn metal elements) system which is free of PbO, has an excellent property against devitrification and has optical constants of a refractive index (Nd) of about 1.58-1.75 and Abbe number (.nu.d) of about 28-45.
As a glass having the above mentioned optical constants, there is known a silicate glass containing PbO. For coping with environmental problems arising in the process of manufacturing a glass, however, various glasses which do not contain a toxic PbO ingredient but instead contain a TiO.sub.2 ingredient have been proposed. For example, the specification of German Patent No. 973350 discloses a silicate glass containing TiO.sub.2 and fluorine and the specification of Japanese Laid Open Patent Publication No. Sho 54-105119 discloses a SiO.sub.2 --TiO.sub.2 --K.sub.2 O--BPO.sub.4 and/or Al(PO.sub.3) .sub.3 system glass. These glasses, however, are insufficient in stability against devitrification. Besides, the former glass is hard to homogenize in melting due to evaporation of the fluorine ingredient. The specification of U.S. Pat. No. 4,812,423 discloses a glass of a SiO.sub.2 --TiO.sub.2 --Li.sub.2 O--K.sub.2 O system. This glass, however, is also insufficient in stability against devitrification.
It is, therefore, an object of the invention to comprehensively improve the disadvantages of the prior art optical glasses and provide an optical glass having the above mentioned optical constants and having an improved property against devitrification.